This invention relates to airfoil flap actuation.
Conventional helicopter flight control is achieved through the use of a swashplate/pitch linkage system driving the individual blades. One purpose of the swashplate, therefore, is to mechanically transmit the pilot's control commands from the nonrotating frame to the rotor. Currently, however, there is a growing interest in the field of automatic control of vibrations transmitted to the helicopter airframe from the rotor at certain multiples (higher harmonics) of the basic rotational frequency. Aerodynamic control forces can be applied to the rotor to reduce these vibration levels by actively changing the pitch of the individual blades; however, the effective frequency range of the hydraulically driven conventional swashplate is low relative to the higher harmonic frequencies. In addition to this problem, the complex nature of the airflow field in the region of the rotor makes it desirable to have the option of varying the control force along the blades--an impossibility in the swashplate system. Thus a need has arisen for an aerodynamic control device which can be mounted in the rotating frame as well as possessing good high-frequency response.
Several basic problems must be surmounted before any progress can be made in the design of such an active device. Firstly, the devices require power which must be transmitted from a source in the nonrotating frame, and secondly the effects of a centripetal acceleration field on the order of hundreds of gravities must be taken into consideration. Both considerations eliminate a priori the use of any type of conventional hydraulic system, both in the transmitting of fluids back and forth between the two frames and in the high pressures which would result from the centripetal acceleration of the fluid. Electrical power, however, can be transmitted to the rotating frame with relative ease through a slipring system. (Indeed, such a system is currently used to power de-icing boots in certain helicopters.) The relatively massive moving parts inherent to electromagnetic motors raise difficulties, once again, due to centrifugal forces.